OS Gemini twin rear carb gas conversion

[mitchilito]

UPDATE!!

I thought I was done with the Gemini 160 - but NO! As I mentioned I have a Gemini 300 coming (I can't wait) and I'm getting geared up to do a gas conversion on it. First thing I want is a suitable Walbr carb for it. Well, it turns out that the perfect carb for the 300 (I believe) is on the 160. As I discussed a few posts above, I have a Walbro with a truly TINY venturi - the WT456 - that I was going to use on a single cylinder OS120. However it should really be perfect for the Gemini 160.

So I ordered a rebuild kit for it, rebuilt it and fit it to the 160. It took a little more work than the existing Walbro because the choke and throttle levers were all wrong for model use. No problem - I machined a set of aluminum arms for it and slapped that baby on there!

It was a really old carb, originally designed for a small 4 cycle weed hacker and I had my doubts that I'd be able to re-purpose it - even with the rebuild. But long story short, I did get it running and running pretty well. I'd say it's an improvement over the first one - but only a little. It's pretty hard to slap a stock carb designed for a different engine and expect it to run exactly right on another, very different engine. Interestingly enough, once I got the needles set a well as I could, the idle was not low enough - even with the butterfly closed all the way. SO: the butterfly disk has a little recess cut out of it to supply air when fully closed (see pic). I pulled the disk out and soldered over the recess to close it completey. The engine would now idle very low (see vid) but still not low enough quite kill the engine! I don't know where the air is coming from to keep it running but run it does - barely!

One last note before we look at the visual aids: I ran it for a good half hour today getting the needles right etc. I started with about 8 ounces of fuel and when I was done I still had a good 5 ounces! The amount of fuel this 26cc engine uses (or doesn't use!) is astounding.

 

[propclock]

Bummer another un obtainable obsolete walbro carb.
Not your fault but it seems every time I find a good sized
model engine carb it seams to be "obsolete" WT 456
Repair kits yes, an actual carb? not available unless I am searching incorrectly.
Thank you for your post and your engine runs wonderfully.

 

[petertha]

@mitchilito that's a nice running engine. You seem to know your way around the carbs & conversions. A long time ago when I was looking at carb sizes I came across this Walbro link. Maybe you've seen already but if not, might provide some food for thought on sizing particularly if certain carb models become extinct.
Then I made a cross plot of venturi/orifice sizes to displacement & superimposed a similar table from Perry (methanol) carbs. Not sure if its real or not but appears to form a trend among the points.

http://www.billetboard.com/showthread.php/9036-Carbs-Venturi-Size-and-Formula-For-Proper-Carb

 

[petertha]

Also came across this quite detailed explanation of Walbro carbs. RC air beater (heli guy), but we won't hold that against him LOL

https://www.helifreak.com/showthread.php?t=326599

 

[mitchilito]

Bummer another un obtainable obsolete walbro carb.
Not your fault but it seems every time I find a good sized
model engine carb it seams to be "obsolete" WT 456
Repair kits yes, an actual carb? not available unless I am searching incorrectly.
Thank you for your post and your engine runs wonderfully.

I suspect a similar carb could be bought new. They are so inexpensive if I had to I would probably buy a WT45. . . Fill in the blank.

I would try to research what piece of lawn equipment it came from. We’re looking for the 4 cycle units.

 

[mitchilito]

I suspect a similar carb could be bought new. They are so inexpensive if I had to I would probably buy a WT45. . . Fill in the blank.

I would try to research what piece of lawn equipment it came from. We’re looking for the 4 cycle units.

Peter, this will keep me busy for a while. Thanks!

 

[mitchilito]

Here’s an amazing resource I found on RC Universe:

RE: DLE 30 carb for 4 st ? an replacement for walbro wt-456-1
Walbro model numbers make no sense. I have no idea what the difference might be. Here is a list of walbro carbs that have an accelerator pump. The pump is a very good thing:

Walbro catalog and here is the list of Wt carbs with accelorator pumps, and hi and low needles and no primers.

6.5 mm with fuel pump return spring (4 stroke)
WT 366-1
WT 456-1

7.95mm
WT 37-1
WT 424-1

9.53mm

WT 35-1
WT 38-1
WT 204-1
WT 357-1
WT 367-1
WT 381-1
WT 385-1
WT 417-1
WT 449-1
WT 450-1

11.11 mm
WT 29-1
WT 356-1
WT 358-1
WT 464-1
WT 516-1
WT 517-1
WT 527-1
WT 613-1

12.7 mm
Wt 513-1
Wt 537-1

13.49mm
Wt 171-1
Wt 172-1
Wt 228-1
Wt 285-1
WT 529-1
Wt 451-1
Wt 480-1
Wt 540-1
Wt 542-1
Wt 555-1
Wt 610-1

If your 30cc liked the carb you have it will be about right for the 1.8 (29cc). If the carb is to big the adjustments get to sensitive, to small and you don't get maximum power.

Here is a link to walbro, click on the series you want and look around.

http://wem.walbro.com/walbro/family2.asp

Look around EBAY for a carb that is 10mm or under and is New Old Stock. They are cheap. You may find your WT 456-1 there. Shipping may be an issue for you though.

Remember you need to run a lot of oil in your mix on a glow conversion, 15 or 20%? Maybe someone can comment on this?

 

[Bentwings]

@mitchilito that's a nice running engine. You seem to know your way around the carbs & conversions. A long time ago when I was looking at carb sizes I came across this Walbro link. Maybe you've seen already but if not, might provide some food for thought on sizing particularly if certain carb models become extinct.
Then I made a cross plot of venturi/orifice sizes to displacement & superimposed a similar table from Perry (methanol) carbs. Not sure if its real or not but appears to form a trend among the points.

http://www.billetboard.com/showthread.php/9036-Carbs-Venturi-Size-and-Formula-For-Proper-Carb

I haven’t seen one of these engines for a long time

 

[Vic101]

Very nice! One point that I wasn't clear on - is this still running on glow plugs? I can't decide if I am seeing a spark plug in there or not.

Spark plugs - 1/4 inch that replace the glow plugs. Fabulous job mitchelito thanks for posting!

View attachment 136750

 

[Bentwings]

Spark plugs - 1/4 inch that replace the glow plugs. Fabulous job mitchelito thanks for posting!

View attachment 136750

I tried to open your attachment but it failed. What carb did you use?

You might look at zenoah Gt80 ignition. Interesting thing zenoah has a twisted intake manifold. Along the way I remember Simone thinking that this was not needed so they made straighter intake runners. This did not work at all. From my experiments in compressible flow when I had some eater exotic for the era software , I found that air with gas in it did not like to turn corners very well . The faster the flow the worse it got. The gas drops out of the air. This occurs in automotive intakes to. So there has been extensive flow studies done by all mags of aftermarket intakes edelbroke has published some studies years ago. I’ve had a number of their manifolds and all wired very well. Anyway the GT 80 has an easy ignition. Ours was flawless right from day one even the magneto version worked well . The electronic was easier starting and had a slightly better idle. I could not see any advance or ****** using a self powered timing light . The carb on this engine had a built in port for crankcase pressure. I can’t remember the spark plug size. It’s 65 miles away and packed in a box so I can’t easily check you might look on the internet. There are 1/4” spark plugs. Many small custom engines use these . Again you might have to do some searching possibly Horizon hobby as that’s where I bough the GT80 they had parts too .

Byron

 

[Bentwings]

I tried to open your attachment but it failed. What carb did you use?

You might look at zenoah Gt80 ignition. Interesting thing zenoah has a twisted intake manifold. Along the way I remember Simone thinking that this was not needed so they made straighter intake runners. This did not work at all. From my experiments in compressible flow when I had some eater exotic for the era software , I found that air with gas in it did not like to turn corners very well . The faster the flow the worse it got. The gas drops out of the air. This occurs in automotive intakes to. So there has been extensive flow studies done by all mags of aftermarket intakes edelbroke has published some studies years ago. I’ve had a number of their manifolds and all wired very well. Anyway the GT 80 has an easy ignition. Ours was flawless right from day one even the magneto version worked well . The electronic was easier starting and had a slightly better idle. I could not see any advance or ****** using a self powered timing light . The carb on this engine had a built in port for crankcase pressure. I can’t remember the spark plug size. It’s 65 miles away and packed in a box so I can’t easily check you might look on the internet. There are 1/4” spark plugs. Many small custom engines use these . Again you might have to do some searching possibly Horizon hobby as that’s where I bough the GT80 they had parts too .

Byron’s he GT80 was soak from the start all of my Zenoah engines were spark either magneto or electronic conversions. I’ve not do large glow engines but a few flyers used them as a rule they worked ok other than use a lot of expensive fuel . I ran smaller glow engine at rediculous speeds . I had the advantage of unlimited nitro , oil and alcohol so I could make any brew needed. More than a few engines blew up or self distuct ed

 

[Vic101]

Spark plugs - 1/4 inch that replace the glow plugs. Fabulous job mitchelito thanks for posting!

View attachment 136750

Apologies, here

is the pic I tried to post.

NB - mitchelito is the author and creator of this masterpiece, not me!

 

[methuselah1]

Apologies, hereView attachment 136839 is the pic I tried to post.

NB - mitchelito is the author and creator of this masterpiece, not me!

little plugs are becoming excessively expensive in the UK, but if you can go from 1/4" to 10mm, the NGK CM6 (as used in weed whackers) is less than 1/5th of the price, which is more like it! The thread is M10 by 1mm fine series. Check Ebay for a tap- you can buy cheap 'cos you won't use it often.

Nice looking engine!

-Andrew UK

 

[mitchilito]

Here she is, where she belongs: in a sweet old-time model airplane. This is the second Sig Rascal 110 she has been. I lost the first one due to radio failure and it nearly broke my heart. I loved the engine/airplane combo so much I just had to recreate it - only this time it's way better with all the cool mods I did to the 160. It's running like a top.

 

[ferincr]

Hello,
Do do make these conversions commercially or can you share details on the intake manifold conversion?
I just managed to get one of these gemini (a 240) that I'd like to convert to gas but I never seen a conversion with the carb at the rear.
I'd appreciate any info you can share.
Thanks,
Fernando
Edit: Never mind, I just realized this thread has few pages (search engine brought me right to the last one. Probably the info I asked for is in the previous pages...

 

[Bentwings]

Interesting mods, thanks for sharing.

My (limited) understanding of the Walbro style pump is they rely on the positive & negative pressure pulses of the 2-stroke engine crankcases to act on the diaphragm. I assume you fed a crankcase line or maybe what was your drain to the pressure inlet port of diaphragm? But isn't a 4-stroke pressure profile quite different than 2-stroke (net positive & lower?). I'm not even sure what opposed twin would be. I mean it obviously works, I'm just curious how. Have you ever pulled the fuel line off to see if it squirts? (ie. fuel pressure strength).

I don't think any of the RC methanol multi-cylinder opposed or radials had exhaust pressure taps feeding fuel tank the way the singles did to addfuel pressure boost. My own opinion is that fuel starving in aerobatics is more related to low / lack of fuel pressure. As the model changes orientation so does the tank & thus reduced hydrostatic head. Usually if anything smaller venturi's draw fuel a bit better, but obviously not enough to overcome the flight regime. That's why the YS systems were so nice in both 2S & 4S.

Do you happen to know roughly how big the venturi orifice of the original OS methanol carb was & how that compares to the gasoline Walbro style?

I vaguely recall reading about the Iron Bay accessory. Not sure but I think maybe similar principle to the Cline regulator? Anyway I found an old link. It says compatible to both methanol & gasoline. Anyways I assume you did not run it on the original OS methanol carb at all? You plumbed it to the Walbro (with its pump/regulator unit removed?)
http://www.ironbaymodelcompany.com/ibmco recreational website_010.htmhttp://saito-engines.info/pumps.html

I agree about the electronic ignition. You can get small spark plugs. I don’t know the thread or a supplier. I YHINK you can still get intake and exhaust pipes I’d get them rm. . Then carefully measure the threads and type of seal I’m sure these can vary and are probably metric you may be simply able to cut the fitting and part of the tube off it’s probably stainless steel and very thin wall . You can probably come up with an adaptor to have made I think it would be best to hav these silver brazed to new pipes . It’s not easy to make bends in this thin wall stuff there is a very low melting point bismuth metal that you can easily melt and pour into sealed tube. You still will need to come up with some kind of bender . That will take some effort as ibfoubtvtherevis a commercial bender that will work. Both tillotson and
Walboro carbs . These do come in different sizes but operate basically the same you will need a very small pressure regulator I think both carbs are float style and depend on crank case pulsing for pressure . I’m not really sure how conversions work basically a very low pressure fuel delivery then the small float controls the volume available similar to standard automotive carb these has two needle valves high and low speed I YHINK I’d pick a carb used on Rc engines You could just build an air box representing a manifold . You may look at plain Venturi carbs

 

[ferincr]

Done it again. Posted a question that was answered in one of the videos posted. Sorry

 

[Bentwings]

Done it again. Posted a question that was answered in one of the videos posted. Sorry
[/QUOT many’s of these tilotson and walboro carbs are really small but there limit they were designed around 2 stroke engines so it would be logical to use half the size I think. Some newer 4 stroke lawn equipment is much smaller engines so maybe one from something like that would work . You may just have to go to a small engine shop and ask or search there . There are model 4 stroke engines for Rc . As much as I deal with Rc I really don’t have experience with them I think many are just Venturi carbs anyway . Not a bad thing as they still follow carb rules

 

[ferincr]

Thanks for your reply, I just bought one of these (the 240 gemini) and I'll be converting it to gas.
I managed to get a walbro wt962 which it says that is for 15-30 cc so I guess that would work.
Now is up to figure out Mitchilito tube bender while I wait for the parts delivery

 

[ferincr]

Just to keep the discussion rolling...
Mechanically speaking, are there any pros/cons of moving the carb to the rear of the engine?

I noticed all (or at least most) glow engines the carb intake is partially (or totally) facing the prop wash. I guess this forces air into it.
And most gas engines have the intake 90 degree to the prop wash ( or some of them the rear carb mounted as Mitchilito did here)

So... is this done to avoid changes on the prop wash to affect mixture therefore smoother runs? or is it just to move the carb weight closer to the CG and minimize the nose momentum? OR number 3 I'm overthinking this?
The only con I can think of is that requires more parts fabrication so I'd like to know the benefits since all the conversions I've seen so far keep the carb (and intake tubes in the original position (just the intake facing straight down.

Sorry if this has a simple answer, I'm not an engineer (or a machinist to be honest) I just have enough tools to get myself in trouble.
Fernando